1. Field of the Invention
The present invention pertains to cathode sputtering apparatus, and more particularly to a method for economically producing sputtering targets by pressure consolidating powdered metal to form a target layer and simultaneously forming a diffusion bond between the target layer and a specially contoured backing plate. The target is thereby formed in a near net shape.
2. Description of the Related Art
Sputtering as a means to deposit thin films of a desired material on a substrate has become important in the manufacture of semi-conductor devices, such as integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputtering target by bombardment of the target with ions.
Since the sputtering process removes sputtering material from the targets, the targets have limited lives and must be replaced periodically. The removal of the material from the targets is not uniform across the exposed surfaces of the targets, and the targets, at the ends of their lives, assume "net shapes" or surface profiles characteristic of the ion source, sputtering material and target shape used. These net shapes typically consist of concentric furrows or "sputtering tracks." Unfortunately, the spent or used targets contain relatively large quantities of expensive, unused target materials. Therefore, it has been proposed to form targets having "near net shapes" to minimize the amount of unused sputtering material left at the end of the target's useful life.
Currently available sputtering targets include a backing plate member attached to the target underside. The backing plate members are typically formed from a less expensive material than the target. Most often, a cooling medium is circulated in heat exchange relationship with the target and its accompanying backing plate to dissipate target heating that results from the sputtering process.
In some sputtering targets, the target is physically soldered to the backing plate. For example, tungsten-titanium targets may be soldered to a copper or austenitic (non-magnetic) stainless steel backing plate using indium, tin-indium or tin-silver alloy. Alternatively, a sputtered or electroplated metallic film may be applied to an interfacial surface or surfaces of either or both the target and the backing plate to enhance the wettability of the bond surfaces being joined by the solder connection. In some cases a silver filled epoxy is used as the bonding agent.
If the sputtering system is operated at high power levels, there is the danger that the solder joint between the sputtering material and the backing plate may melt or deteriorate due to excessive heating of the target/backing plate assembly. Cracking or other failure of the sputtering target in this manner results in needless downtime of the equipment. Accordingly, there is a need to provide for a durable target/backing plate bond so that the backing plate may maintain adequate surface contact with the target to ensure proper heat exchange from the target to the cooling medium. If a suitable solder bond is not formed, the target will be limited to low power applications.
In some target assemblies using tungsten-titanium sputtering material, the deformation of the target assembly is minimized by incorporating a backing plate made of titanium which has a thermal expansion rate that is closely matched to that of the sputtering material. It is extremely difficult to produce a reliable solder bond with the titanium backing plate, however, because of the non-wetting characteristics of titanium.
Boys, U.S. Pat. No. 5,215,639, proposes composite sputtering target structures in which the sputtering material has a profiled, curvilinear, back surface conforming in shape to the eroded target surface at end-of-life. A backing plate is bonded to the sputtering target for support; the backing plate having a bonded surface complementary to the end-of-life shape of the sputtering target and designed to mate therewith. In one disclosed alternative, the sputtering material is provided in powder form and the backing plate and target are joined by compression bonding, preferably isostatic pressing.
It is desirable to form a flat, even surface on the finished sputtering target. Indeed, the precision with which the near net shape of the target is realized depends on the evenness of the initial exposed surface. It has been found that sputtering surfaces formed on targets produced by methods such as Boys' are often marred by defects such as concentric depressions corresponding to the sputtering rings of the near net profiles on the surfaces of the backing plates. Accordingly, targets produced by methods such as Boys' may require an unnecessary amount of additional machining to produce a desired shape. Therefore, there remains a need in the art for a method for efficiently producing sputtering targets with uniformly even sputtering surfaces.